Copolymers of a diallylamine, such as a diallyldialkylammonium chloride and a diallylalkyl amine hydrochloride and sulfur dioxide are produced industrially because they can be produced easily by copolymerizing such monomers, and there has been proposed to use them in the fine chemical fields, such as water-soluble paints and color fastness improvers for dyed goods. (See, for example, Patent Literature 1.)
As a method of obtaining a polyaminesulfone by copolymerizing a hydrochloride salt of a diallylamine and sulfur dioxide with a radical initiator, a polar solvent is used as described in Patent Literature 1. Although water is mainly used as the polar solvent in order to maintain solubility after polymerization or because the raw material hydrochloric acid is an aqueous solution, the average molecular weight of the resulting copolymer is several thousand at most when water is used as the solvent in conventional technologies.
On the other hand, although it is known that a copolymer of a diallylamine and sulfur dioxide having a high molecular weight is obtained by making the monomer concentration extremely high and performing polymerization with light even when using water as a solvent, it is difficult to carry out the polymerization quantitatively, and the operation becomes complicated, for example, pulverization becomes necessary in order to divide the resulting polymer into small portions because the resulting polymer is massive, and therefore, it was difficult to produce a copolymer having a high molecular weight industrially at low cost even by this method.
With rising global environmental awareness, various chemicals used in the fine chemical field have been required to be free of halogens, such as chlorine, and there are demand for copolymers of a salt other than diallylalkylamine hydrochlorides and sulfur dioxide instead of copolymers of a diallylalkylamine hydrochloride and sulfur dioxide.
Although a copolymer of a diallylamine and sulfur dioxide with high solubility in organic solvents without using chlorine has been synthesized by performing the synthesis using, for example, an alkylsulfate as an addition salt instead of the diallylalkylamine hydrochloride (see, for example, Patent Literature 2), the solubility that having been confirmed is only 2 wt. % at most and this is based on the powder obtained after the polymerization.
Although a diallylamine and sulfur dioxide copolymerize through formation of a charge transfer complex, sulfur dioxide partly yields sulfurous acid in a water solvent. It is commonly known that sulfurous acid is used as a chain transfer agent in polymerization. Therefore, when using a water solvent, it is difficult to obtain a copolymer with a high molecular weight and, for example, in the case of agitation polymerization, merely a copolymer having an average molecular weight of several tens of thousands at most is obtained by a method including addition of an acid (see, for example, Patent Literature 3).
On the other hand, in syntheses using an organic solvent, the resulting copolymer is insoluble in the organic solvent and, therefore, the copolymer precipitates rapidly in the form of a highly viscous paste form, falling in a state where it is difficult to control agitation cooling.
Although dimethyl sulfoxide and dimethylformamide are known as solvents excellent in solubility as disclosed in Patent Literature 1, dimethyl sulfoxide reacts with SO2 to produce dimethyl sulfide and SO3 and inhibits polymerization. For this reason, at a charge molar ratio of SO2 to a diallylamine of about 1:1, which is intrinsically suitable for obtaining a copolymer having a high molecular weight, it is difficult to obtain a polyaminesulfone with a high degree of polymerization due to the influence of SO3 generated from SO2, which does not participate in polymerization. Moreover, dimethyl sulfide is a malodorous substance and has a possibility of worsening working environment. Dimethylformamide is decomposed into formic acid and dimethylamine under an acidic condition, which is the polymerization condition. Therefore, it is hard to consider dimethyl sulfoxide and dimethylformamide as proper and practical solvents.
For this reason, synthesis using an organic solvent has not been put in practical use.
When using water as a solvent, a charge transfer complex of a diallylamine and sulfur dioxide in water decomposes at high temperatures of 20° C. or higher and sulfur dioxide flows out of the system, resulting in drop of yield. For this reason, it was necessary to use an organic peroxide or an inorganic peroxide as an initiator for a copolymerization reaction in water and perform copolymerization while controlling the system mainly at a low temperature of 30° C. or lower utilizing a redox reaction with sulfurous acid, which has been by-produced by sulfur dioxide. Although this reaction can be applied also to organic solvents, it is difficult to obtain a copolymer with a high molecular weight because polymerization by a redox reaction leads to a large decomposition amount of the initiator.